Plastic film made by a film casting article

ABSTRACT

A METHOD OF MAKING A FILM CASTING ARTICLE HAVING A CASTING SURFACE CAPABLE OF IMPARTING TO POLYMERIC FILM CAST THEREON AND STRIPPED THEREFROM DESIRABLE OPTICAL AND SLITROLL FORMATION PROPERTIES. THE CASTING SURFACE OF THE FILM CASTING ARTICLE IS POLISHED, BLASTED WITH GRIT, AND STONED AND POLISHED AGAIN TO IMPART CRITICAL &#34;ROUGHNESS&#34; CHARACTERISTICS TO SUCH CASTING SURFACE.

Aug. 14, 1973 T. D. STIEGLER ET AL PLASTIC FILM MADE BY A FILM CASTING ARTICLE Filed Sept. 8, 1971 FIGJ INVENTORS THEODORE DONALD STIEGLER CLYDE GEORGE LEARN, SR.

ATTORNEY United States Patent 3,752,731 PLASTIC FILM MADE BY A FILM CASTING ARTICLE Theodore Donald Stiegler and Clyde George Learn, Sr., Richmond, Va., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del.

Original application Dec. 2, 1969, Ser. No. 881,370, now abandoned. Divided and this application Sept. 8, 1971, Ser. No. 178,697

Int. Cl. D06n 7/04 US. Cl. 161-164 1 Claim ABSTRACT OF THE DISCLOSURE A method of making a film casting article having a casting surface capable of imparting to polymeric film cast thereon and stripped therefrom desirable optical and slitroll formation properties. The casting surface of the film casting article is polished, blasted with grit, and stoned and polished again to impart critical roughness characteristics to such casting surface.

This is a division of application Ser. N0. 881,370, filed Dec. 2, 1969, now abandoned.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention is a film casting article and, additionally, is a method of making a film casting article having a novel metallic casting surface on which polymeric material can be cast into film. The film casting surface is roughened, in a novel manner, whereby film cast thereon has imparted to it desirable surface physical properties.

(2) Description of the prior art Roughened casting surfaces have long been known to the art. US. Pat. 2,866,717 to Bristol, which is exemplary of such prior art, shows a process for preparing a film casting surface having from 100 to 2000 microscopic pits per square centimeter whose radii range from 1 to 10 microns which comprises coating a casting base with a cement comprising polyvinyl alcohol dissolved in a solvent.

US. Pat. 3,063,763 to Zubrisky shows a chromium bea1= ing surface having a multitude of minute projections and depressions which correspond in size and shape to projections and depressions formed in a surface of solid unfissured chromium by blasting the surface with a hard abrasive grit ranging in size from 60 to 150 mesh.

US. Pat. 3,177,558 to Gronholz et al. discloses a highly polished film casting surface having from 2x10 to 12x10 randomly spaced hemi-spherical indentations per square inch of surface with the indentations having diameters within the range of 50 to 1500 microinches.

US. Pat. 3,310,860 to Gronholz et a1. shows a process for producing nodular chill rolls" having a chromium plated surface having an over-all roughness of 4 to 20 microinches due to the presence of 2x10 to 12 10 hemi-spherical indentations per square inch of surface having diameters of 50 to 1500 microinches.

Canadian Pat. No. 536,619 to Bristol shows a process for manufacturing non-blocking film having uniformly distributed microscopic bumps on its surface by casting a film-forming solution onto a casting surface having 100 to 2000 microscopic pits per square centimeter with the radii of the pits generally ranging from about 1 to 10' microns.

Canadian Pat. No. 572,487 to Bristol shows a nonblocking thermosealable film web having about 100 to 2000 bumps per square centimeter whose radii generally 3,752,731 Patented Aug. 14, 1973 ice SUMMARY OF THE INVENTION This invention is a film casting article and a method of making such article having a casting surface on which molten polymeric material can be cast to form a novel plastic film with good visual, winding and other properties.

Briefly described, such method includes the steps of forming a metallic casting surface having normal minute surface pits therein, polishing the casting surface until the pits in the surface are less than 60 microinches deep, blasting the polished casting surface with hard abrasive grit to form a multitude of new pits in the surface which are spaced an average of less than 0.004 inch apart and finishing the grit blasted surface until the pit depths are less than 60 microinches and the nominal diameters of the pits are no more than 0.0005 inch.

In making thermoplastic polymeric film, generally a molten polymer or a polymeric solution is extruded through a long narrow die orifice onto a moving highly polished cooled metallic casting surface of a film casting article or member. The uniformity of the gauge across the cast film or web is determined largely by the setting of the two die lips which form the long narrow die orifice. The temperature of the cooled casting surface on which the extruded polymer is cast largely determines the quenching rate and the finish of such casting surface essentially controls the surface characteristics imparted to the film web cast thereon. The film casting member is usually in the form of a cylinder although certain filmmaking methods employ cooled metallic belts.

Many film uses, such as in cellulose acetate film windows in fioral containers and in certain envelopes, require high transparency and clarity properties in the cast film. Such film characteristics are usually achieved by employing highly finished metallic casting surfaces. Super-finished casting surfaces, however, can cause problems, particularly relating to the winding of the web into roll form after it is slit into the desired web widths.

By practicing the method of this invention, this winding problem is controlled or alleviated by roughening the casting surface of a casting member so that the cast image of the film cast thereon has a controlled degree of surface roughness. This degree of roughness controls or determines the slip and anti-blocking or winding properties of the cast film; however, it may also adversely affect the clarity of the cast film.

Cellulose acetate and polyethylene terephthalate films made for use in the recording tape industry require high clarity of the film and other good optical properties. Further, good slit-roll formation properties of the film is critical since the one-side coated webs are slit into very narrow widths and wound onto narrow cores at high production rates. The desired combination of film properties for this use area points up the necessity for a film casting surface which will impart to chill-roll cast polymeric film excellent optical clarity with a surface roughness which is not noticeable to the human eye but is suflicient to insure good roll formation of re-slit rolls.

The metallic casting surface made by the method of this invention with its many critically formed pits or indentations produces a reverse-replica surface on polymeric film that is melt or solution cast thereon thereby forming a multitude of microscopic protrusions or projections on the film surface stripped from the casting surface. These protrusions cause the proper amount of air to be held between wound film layers as the film web is wound to prevent blocking and also interlock sufficiently Within the wound web layers to cause the alignment of the winding web to be maintained. The microscopic size of the protrusions, on the other hand, are such that the clarity of the cast film is not adversely affected or is maintained within acceptable limits.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary diagrammatic plan view of a film casting article made by the method of the present invention, the surface of which has been magnified approximately 250 times, and

FIG. 2 is a typical magnified sectional elevation, in diagrammatic form, of a film casting article embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, a novel film casting article or member of this invention consists of an article surface portion or part 11 having a roughened film casting surface 12, which surface portion 11 is afiixed to or integral with or deposited upon an article base portion 13.

The casting surface 12 may be of chromium, stainless steel or nickel metals. A casting surface 12 of metallic nickel is preferred. This can be attained by using a nickel body or by using a metallic body such as finished steel on which is deposited a first layer of copper and a second layer of nickel.

The article surface portion 11 may be a layer of nickel which is deposited on a smoothly finished copper-covered surface, for example, of the article base portion 13 which, in turn, may be in the form of a cylindrical roll or a flat surface, such as the upper run of an endless metallic belt.

The film casting surface 12 is roughened in a novel manner and contains a multitude of pits or irregularly shaped indentations 14 which preferably have a depth, measured from peak to valley, ranging from 25 to 60 microinches, an average diameter of less than 0.0005 inch and an average spacing of less than 0.0045 inch. The pits 14 are highly irregular in cross-sectional shape and are randomly distributed across the casting surface 12. In addition to these pits 14, there is the unavoidable presence of microscopic scratches 15 remaining from the various machine operations. These are finished such that no individual scratch 15 exceeds more than microinches in depth and all such scratches average no more than 6 microinches in depth.

METHOD OF MAKING THE FILM CASTING MEMBER The preferred method for making the film casting member 10 with its novel film casting surface 12 includes the steps of depositing by electrolytic deposition up to 0.025 inch layer of nickel onto a previously finished metallic surface of the base portion 13 of the casting member 10 thereby to form the surface portion 11 of such casting member 10; stoning the nickel casting surface 12 of the surface portion 11 until the surface pits 14 in the nickel surface are less than 60 microinches deep, all accumulated scratches 15 on or in the surface 12 arithmetically average no more than 6 microinches in depth and no individual scratch 15 exceeds more than 20 microinches in depth; blasting the finished nickel casting surface 12 using hard abrasive grit to form a multitude of new pits 14 on the metallic nickel surface 12 which are spaced an average of less than 0.004 inch apart and finish honing of the pitted nickel surface 12 until the pit depths are from to 60 microinches deep and the nominal diameters of the pits 14 are substantially no more than 0.0005 inch.

It is preferred that the nickel electroplating be done on a machined surface having a roughness not exceeding 10 microinches, arithmetic average, with an average variation less than 2 microinches and that the electroplating be accomplished in a conventional two-step method comprising an electrodeposition of a layer of copper over a steel body and a layer of nickel over the deposited copper layer. The specific method steps for this electroplating are well known in the art.

The grit blasting of the nickel surface is accomplished by entraining from to 320 mesh aluminum oxide grit within a compressed air stream and propelling this stream against the nickel casting surface from an approximate inch diameter orifice while employing approximately p.s.i. of air pressure. In the preferred embodiment 320 mesh aluminum oxide particles were employed with an air gun pressure of 113 p.s.i. and two passes over the surface were taken. These were at 2 inches per minute and at 1% inches per minutes while maintaing a 6-inch distance between the gun and the casting surface. The speed and number of passes may be varied to produce the desired and critical surface conditions.

Tiny ridges and certain surface irregularities which are left by the grit-blasting operation are removed or minimized by the honing operation. The grit-blasted surface is honed preferably using a honing stone that is molded from aluminum oxide using a resinoid or vitrified bond. This honing step is primarily a surface-refining operation which is intended to correct the high peaks and tiny ridges which sometimes result from the grit-blasting operation.

The following examples demonstrate the variations in pitted surfaces which can be attained by varying the size of the grit used and the number of passes to which the casting surfaces were subjected.

EXAMPLES 1 TO 9 The surfaces of cylindrical casting rolls 30 inches in diameter were electroplated with layers of metallic nickel to a depth of 0.025 inch by electrolytic deposition. These surfaces were smoothly finished so that all surface pits were less than 60 microinches deep and so that the average surface scratch depth was below 6 microinches. They were then grit blasted using aluminum oxide grit of 80, 120, 180, 240 and 320 mesh size. The grit was propelled against the nickel surfaces using 113 p.s.i. compressed air passing through a A-inch nozzle held 6 inches from the nickel surface. The number of passes of the nozzle did not exceed two.

The following table shows the characterization of the grit-blasted nickel surfaces in terms of the pit depths, pit widths and pit spacings.

Legend for dimension symbol and spacing code:

u=Microinch or 0.000001 inch.

1 =Pit. spacing satisfactory.

2=Pit spacing too far apart.

3=Pit spacing too close.

Examples 3, 7 and 9 yielded roll surfaces that were satisfactory for continued finishing by honing until the preferred previously enumerated final roll surface characterizations were achieved. Examples 1 and '8 were usable surfaces but required an excessive amount of stoning time to achieve the final desired surface characteristics. The pit spacings of Examples 2, 4, 5 and 6 were too close for rolls having final optimum finish.

The final casting surfaces which resulted from honing the rolls of Examples 3, 7 and 9 had pit depths of from 50 to 60 microinches, pit spacing of less than 0.004 inch and average pit sizes of 500 microinches or less.

Cellulose acetate polymer was directly solvent cast onto these surfaces and the film so cast was examined for surface characteristics and for slit-roll winding characteristics. It was found in all cases that the films cast on these surfaces had acceptable clarity together with excellent slit-roll formation properties.

What is claimed is: 1. A plastic film made by casting polymeric material onto a film casting surface of a film casting article made by a method including the steps of:

depositing a layer of nickel of from 0.005 inch to 0.025 inch thickness onto an article base portion whereby to form a smooth metallic film casting surface having normal minute surface pits and scratches therein;

polishing the casting surface until the depths of the surface pits are less than 60 microinches, all scratches average no more than 6 microinches in depth and no individual scratch exceeds more than 20 microinches in depth;

blasting the polished casting surface with hard abrasive grit to form a multitude of pits on the metallic surface, which pits are spaced an average of less than 0.004 inch apart;

References Cited UNITED STATES PATENTS 12/1958 Bristol 117--41 X 11/1962 Zubrisky 308241 3/1967 Gronholz 29-148.4 D 11/ 1968 Markovic 29-148.4 D

GEORGE F. LESMES, Primary Examiner P. C. IVES, Assistant Examiner US. Cl. X.R.

l829; 29-148.4 D; 5l326; 117-8, 10, 41; 204-36 

